Mitosis

Division into two identical cells

Mitosis is the name for the way that a
cell duplicatesitself so that each daughter cell receives an
identical copy of its genetic material. At the end of mitosis, there
will be two cells instead of one. They will be identical to each other.

The events that occur differ in prokaryotes and in eukaryotes.

Before we start describing the different phases of this cell
division, we need to remember what a cell
looks like in its normal state, which is known as the interphase. A
normal eukaryotic cell has pairs of chromosomes (2n chromosomes).
During this phase, DNA is being replicated.
At the end of interphase, the amount of DNA will be twice the
normal amount. For a transient moment the cell will have 4n
chromosomes.

Cell division

The act of cell division is normally a single process, but it can
be broken down into a number of "phases". It is convenient to give a
name to each phase:

Prophase

Prophase is the first stage of cell division. Here,
the cell prepares itself for division. The nucleus swells,
and chromosomes become visible.
Each chromosome has two chromatids as a result of duplication of the DNA which took place during interphase. The two chromatids are linked together at a
centromere. The centrosome (2 centrioles) duplicates into 2 diplosomes, and each diplosome, or aster moves toward opposite
poles of the nucleus.

Metaphase:

Microtubules assemble, and form a network (the spindle
fibres). The chromosomes move towards the equator of the
cell, where they are visible. This is the phase in which
morphological studies of chromosomes are carried
out, often for clinical purposes.

Anaphase

The two sister chromatids separate.Each one migrates
to opposite ends of the cell. So each daughter cell has an identical complement of chromosomes
. The nuclear membrane has disappeared at
this stage. The cell membrane expands as the cell itself
elongates. The diameter of the cell decreases at the
equator.

Telophase

A new membrane forms around the new nuclei and two cells
are quickly formed. The chromatid, now called a chromosome, uncoils,
and the nucleolus becomes visible again.

Each cell contains a pair of chromosomes (2n chromosomes)

Some remarks!

What triggers mitosis?

External signals, hormones, internal factors, growth factors (proteins able to trigger mitosis): these proteins have a receptor on
the cell membrane. When the protein is attached to the receptor, it
triggers several events in the cell that end with the division
of the cell.

What is Cancer?

A cell becomes cancerous when:

it loses the ability to stop dividing when
encountering another cell (hence the formation of a tumour),

it becomes immortal.

Conclusions

One cell with 2n chromosomes divides to create two cells with 2n
chromosomes :the number of chromosomes per cell is conserved.

The replication of DNA preceding the division of the cell has
prepared chromosomes with two chromatids. The material has been
multiplied by two before the division takes place: the quantity of
material per cell is conserved.

The replication of DNA results in two identical chromatids:
the quality is conserved.

In the chromosome cycle, from interphase to mitosis, the same
material, goes from being in a diffuse form (during interphase)
to being in a condensed form (as chromatids, during some phases of
the mitosis). Only the condensed material can be separated.